In recent years, advances in the image processing technology have achieved higher image quality in video cameras for capturing and recording images and television receivers for displaying captured images, and have made it possible for present video cameras and television receivers to reproduce sharper images than conventional video cameras and television receivers.
FIG. 1 of the accompanying drawings shows an AV (Audio Visual) system including a video camera and a television receiver. In FIG. 1, a signal of an image captured by a video camera 1 is supplied through a recording medium 11 or a network 12 to a television receiver 2, which displays the image captured by the video camera 1.
Details of the video camera 1 and the television receiver 2 will be described below with reference to FIGS. 2 and 3 of the accompanying drawings.
FIG. 2 shows in block form details of the video camera 1 illustrated in FIG. 1. The video camera 1 performs its processing operation according to predetermined standards (e.g., ITU-R (International Telecommunication Union Radiocommunication sector) BT (Broadcasting service (Television)).601 (hereinafter referred to as BT.601) or ITU-R BT.709 (hereinafter referred to as BT.709)). It is assumed here that the video camera 1 performs its processing operation according to BT.709.
In FIG. 2, the video camera 1 includes a operation unit 21, an image capturing unit 22, an A/D converter 23, a primary color converter 24, a color signal corrector 25, a photoelectric transducer 26, a color signal converter 27, an encoder 28, a controller 29, a recorder 30, and a communication unit 31.
The operation unit 21 is operated by the user when the user enters various commands into the video camera 1. The operation unit 21 supplies signals representing the execution of processing sequences that are indicated by the operation by the user of the operation unit 21, to blocks that perform the respective processing sequences. For example, the operation unit 21 supplies the image capturing unit 22 with a signal representing the capturing of an image, and also supplies the controller 29 with a signal representing a destination of a signal (hereinafter referred to as an image signal) indicative of an image captured by the image capturing unit 22.
The image capturing unit 22 starts or stops an image capturing process according to an instruction from the operation unit 21. The image capturing unit 22 supplies an image signal indicative of an image captured by the image capturing unit 22 to the A/D (Analog/Digital) converter 23. The image capturing unit 22 includes a CMOS (Complementary Metal Oxide Semiconductor) imager, a CCD (Charge Coupled Device), or the like, and outputs color signals R, G, B (Red, Green, Blue) as an image signal.
The A/D converter 23 converts analog color signals supplied from the image capturing unit 22 into digital color signals, and supplies the digital color signals to the primary color converter 24. The color signals R, G, B that are supplied from the A/D converter 23 to the primary color converter 24 are referred to as color signals Rorg, Gorg, Borg.
The primary color converter 24 converts the color signals Rorg, Gorg, Borg supplied from the A/D converter 23 into color signals R709, G709, B709 based on primary colors under BT.709, and supplies the color signals R709, G709, B709 to the color signal corrector 25. Specifically, the primary color converter 24 converts the color signals Rorg, Gorg, Borg supplied from the A/D converter 23 into color signals R709, G709, B709 based on primary colors under BT.709 according to the following equation (1):
                              (                                                                      R                  709                                                                                                      G                  709                                                                                                      B                  709                                                              )                =                              (                                                            1.5968                                                                      -                    0.6351                                                                    0.0383                                                                                                  -                    0.1464                                                                    1.2259                                                                      -                    0.0795                                                                                                                    -                    0.0141                                                                                        -                    0.1086                                                                    1.1227                                                      )                    ⁢                      (                                                                                R                    org                                                                                                                    G                    org                                                                                                                    B                    org                                                                        )                                              (        1        )            
The matrixes of the equation (1) differ depending on the primary color points of the image capturing unit 22.
The color signal corrector 25 corrects the color signals R709, G709, B709 supplied from the primary color converter 24 into color signals R709, G709, B709 in a numerical range from 0 to 1.0 defined according to BT.709. Specifically, the color signal corrector 25 corrects color signals R709, G709, B709 which are smaller than 0 into 0, i.e., clips color signals R709, G709, B709, and corrects color signals R709, G709, B709 which are greater than 1.0 into 1.0, and supplies the corrected color signals R709, G709, B709 to the photoelectric transducer 26. It is assumed that the numerical values 0, 1.0 of the numerical range from 0 to 1.0 are minimum and maximum values, respectively, of the color signals R709, G709, B709 according to BT.709.
The photoelectric transducer 26 converts the color signals R709, G709, B709 supplied from the color signal corrector 25 into color signals R′709, G′709, B′709 that are corrected with the γ of a display mechanism of B.709 (the nonlinearity of light emission luminance with respect to the image signal) according to photoelectric transducer characteristics according to BT.709, and supplies the converted color signals R′709, G′709, B′709 to the color signal converter 27.
Specifically, the photoelectric transducer 26 converts the color signals R709, G709, B709 into color signals R′709, G′709, B′709 according to the following equation (2) and supplies the converted color signals R′709, G′709, B′709 to the color signal converter 27:R′709=1.099×(R709)0.45−0.099 0.018≦R709≦1.0 R′709=4.5×R709 0≦R709<0.018  (2)
The photoelectric transducer characteristics between the color signal R709 and the color signal R′709 are defined in the range from the minimum value to the maximum value of the color signal R709 according to BT.709, i.e., in the range from 0 to 1.0. The photoelectric transducer characteristics between the color signal G709 and the color signal G′709 and the photoelectric transducer characteristics between the color signal B709 and the color signal B′709 are also similarly defined.
The color signal converter 27 converts the color signals R′709, G′709, B′709 supplied from the photoelectric transducer 26 into a luminance signal Y′709 and color difference signals Cb′709, Cr′709 under BT.709 according to the equation (3) shown below, and supplies the luminance signal Y′709 and the color difference signals Cb′709, Cr′709, each expressed in 8 bits, to the encoder 28.
                              (                                                                      Y                  709                  ′                                                                                                      Cb                  709                  ′                                                                                                      Cr                  709                  ′                                                              )                =                              (                                                            0.2126                                                  0.7152                                                  0.0722                                                                                                  -                    0.1146                                                                                        -                    0.3854                                                                    0.5000                                                                              0.5000                                                                      -                    0.4542                                                                                        -                    0.0458                                                                        )                    ⁢                      (                                                                                R                    709                    ′                                                                                                                    G                    709                    ′                                                                                                                    B                    709                    ′                                                                        )                                              (        3        )            
The matrixes of the equation (3) are matrixes prescribed for 1125/60/2:1 Signal Format under BT.709.
According to BT.709, the luminance signal Y′709 generated by the color signal converter 27 according to the equation (3) is of value in a numerical range from 0 to 1.0. Each of the color difference signals Cb′709, Cr′709 generated by the color signal converter 27 according to the equation (3) is of value in a numerical range from −0.5 to 0.5.
Furthermore, the color signal converter 27 assigns the luminance signal Y′709 in the numerical range from 0 to 1.0, which is generated by the color signal converter 27 according to the equation (3), to an integral value in an integral range from 16 to 235 which is smaller than an integral range from 0 to 255 that can be expressed in 8 bits, and supplies the luminance signal Y′709 that is assigned to the integral value as a luminance signal according to BT.709 to the encoder 28. The color signal converter 27 assigns each of the color difference signals Cb′709, Cr′709 in the numerical range from −0.5 to 0.5, which is generated by the color signal converter 27 according to the equation (3), to an integral value in an integral range from 16 to 240 which is smaller than the integral range from 0 to 255 that can be expressed in 8 bits, and supplies the color difference signals Cb′709, Cr′709 that are assigned to the integral value as color difference signals according to BT.709 to the encoder 28.
The encoder 28 encodes the luminance signal Y′709 and the color difference signals Cb′709, Cr′709, each in 8 bits, supplied from the color signal converter 27 according to a predetermined format, for example, such as MPEG (Moving Picture Experts Group) and supplies the resultant encoded data to the controller 29.
The controller 29 supplies the encoded data supplied from the encoder 28 to the recorder 30 or the communication unit 31 according to an instruction from the operation unit 21.
The recorder 30 records the encoded data supplied from the controller 29 in the recording medium 11 shown in FIG. 1. The communication unit 31 transmits the encoded data supplied from the controller 29 through the network 12 shown in FIG. 1.
FIG. 3 shows in block form the television receiver 2 shown in FIG. 1. The television receiver 2 performs its processing operation according to predetermined standards (e.g., BT.601 or BT.709). It is assumed here that the television receiver 2 performs its processing operation according to BT.709.
As shown in FIG. 3, the television receiver 2 includes an image signal input unit 41, a luminance and color difference signal converter 42, an inherent γ characteristics corrector 43, a D/A converter 44, and a display mechanism 45.
The image signal input unit 41 receives encoded data reproduced from the recording medium 11 or transmitted from the network 12. The image signal input unit 41 also decodes the encoded data according to a predetermined format such as MPEG, for example, and supplies a luminance signal Y′709 and color difference signals Cb′709, Cr′709, each expressed in 8 bits, according to BT.709, which are produced from the decoded data, to the luminance and color difference signal converter 42.
The luminance and color difference signal converter 42 converts the luminance signal Y′709 and the color difference signals Cb′709, Cr′709 supplied from the image signal input unit 41 into color signals R′709, G′709, B′709 under BT.709 according to the equation (4) shown below, and supplies the color signals R′709, G′709, B′709 to the inherent γ characteristics corrector 43.
                              (                                                                      R                  709                  ′                                                                                                      G                  709                  ′                                                                                                      B                  709                  ′                                                              )                =                              (                                                            1.0000                                                  0.0000                                                  1.5747                                                                              1.0000                                                                      -                    0.1873                                                                                        -                    0.4682                                                                                                1.0000                                                  1.8556                                                  0.0000                                                      )                    ⁢                      (                                                                                Y                    709                    ′                                                                                                                    Cb                    709                    ′                                                                                                                    Cr                    709                    ′                                                                        )                                              (        4        )            
The luminance signal Y′709 according to BT.709 which is supplied from the image signal input unit 41 to the luminance and color difference signal converter 42 is of an integral value in an integral range from 16 to 235 which can be expressed in 8 bits, as described above. Each of the color difference signals Cb′709, Cr′709 according to BT.709 which are supplied from the image signal input unit 41 to the luminance and color difference signal converter 42 is of an integral value in an integral range from 16 to 240 which can be expressed in 8 bits, as described above.
The luminance and color difference signal converter 42 sets the luminance signal Y′709 of the integral value in the integral range from 16 to 235, which is supplied to the luminance and color difference signal converter 42, to a value in a numerical range from 0 to 1.0, and also sets each of the color difference signals Cb′709, Cr′709 of the integral value in the integral range from 16 to 240 to a value in a numerical range from −0.5 to 0.5. The luminance and color difference signal converter 42 also converts the luminance signal Y′709 expressed in the numerical range from 0 to 1.0 and the color difference signals Cb′709, Cr′709 expressed in the numerical range from −0.5 to 0.5 into color signals R′709, G′709, B′709 according to the equation (4).
If the γ characteristics of the display mechanism 45 of the television receiver 2 are different from the photoelectric transducer characteristics (γ characteristics) represented by the equation (2) of BT.709, then the inherent γ characteristics corrector 43 converts the color signals R′709, G′709, B′709 supplied from the luminance and color difference signal converter 42 into color signals R′709, G′709, B′709 according to the inherent γ characteristics of the display mechanism 45 (CRT (Cathode Ray Tube) or the like) of the television receiver 2, and supplies the color signals R′709, G′709, B′709 to the D/A converter 44.
If the γ characteristics of the display mechanism 45 of the television receiver 2 are identical to the photoelectric transducer characteristics of BT.709, then the inherent γ characteristics corrector 43 is not required.
The D/A converter 44 converts the digital color signals R′709, G′709, B′709 supplied from the inherent γ characteristics corrector 43 into analog color signals R′709, G′709, B′709, and supplies the analog color signals R′709, G′709, B′709 to the display mechanism 45.
The display mechanism 45 includes a CRT or the like, and displays an image based on the color signals R′709, G′709, B′709 supplied from the D/A converter 44.
The color signals, the luminance signals, and the color difference signals according to BT.709 which are processed in the video camera 1 and the television receiver 2 are prescribed in RECOMMENDATION ITU-R BT.709-4.
FIG. 4 of the accompanying drawings shows the chromaticity coordinate positions of primary colors and reference white in the CIE (Commission Internationale de I'Eclariage) colorimetric system.